1. Field of the Invention
The present invention relates to a manufacturing method for optical couplers/splitters and to a method for adjusting the optical characteristics of planar lightwave circuit devices. In particular, the present invention relates to a tap coupler device for splitting signal light from a communication circuit and entering it into a monitoring photodiode.
2. Description of the Related Art
As conventional optical couplers/splitters, optical fiber couplers/splitters and planar lightwave circuits (abbreviated to PLC) have been proposed.
There are various configurations of PLC devices such as, for example, Y-shaped optical waveguide cores, Mach-Zehnder interferometers, and arrayed waveguide grating (abbreviated to AWG) which are used as the optical couplers/splitters or optical multiplexers/demultiplexers.
In the optical fiber couplers/splitters, because the adjustment of optical characteristics such as the branching ratio and the like is comparatively simple during the manufacturing process, the design and manufacturing of the optical fiber couplers/splitters having various branching ratios is possible, so that this type of optical fiber couplers/splitters are suitable for large item small volume production. On the other hand, because they are formed from plural optical fibers, it is difficult to integrate these devices.
In contrast to this, the PLC devices are suitable for highly integrated optical devices. However, although PLC devices are suitable for mass production of devices having identical characteristics, they have the problem of not being suited for large item small volume production.
FIG. 6A is a plan view showing an example of a conventional PLC device. FIG. 6B is a cross sectional view along the line A—A in FIG. 6A.
The symbol 2 in the drawings indicates a substrate. A cladding layer 3 is formed on this substrate 2, and Y-shaped waveguide core 4 is provided inside the cladding layer 3.
A silicon substrate or the like, for example, may be used for the substrate 2. The cladding 3 and the Y-shaped waveguide core 4 may be formed, for example, from silica glass.
The Y-shaped waveguide cores 4 is formed from a material having a higher refractive index than the cladding layer 3 in order for the Y-shaped waveguide core 4 to act as an optical wave guide. If the cladding layer 3 is formed, for example, from pure silica glass, germaniun doped silica glass material or the like is used for the Y-shaped waveguide core 4.
The Y-shaped waveguide core 4 is formed in such a way that a core having a cross-sectional shape of a square-rod is split at a portion along length of the core into two branch cores. The Y-shaped waveguide cores 4 is comprised by an input-side linear section 4a extending from the input side; a branching section 4b which is formed on the output side of the input-side linear section 4a and is formed by gradually widening the input-side linear section 4a; a separation section 4c formed in curved or linear shape so that two branched cores 5a and 5b separate from each other as they extend from the branching section 4b; and an output-side linear section 4d in which the branched cores 5a and 5b are disposed in parallel to each other.
In the branching section 4b, the input-side linear section 4a extends from the input end of the branching section 4b, and the branching cores 5a and 5b extend from the bottom perimeter opposite to the input end of the branching section 4b. 
The port 6a on the input side of the Y-shaped waveguide core 4 and two ports 6b and 6c on the output side of the Y-shaped waveguide core 4 are placed on the same flat plane that is also parallel to the bottom and top surfaces of the substrate 2.
When signal light is inputted into the port 6a, the signal light is outputted from both the ports 6b and 6c after being split into two branches at a predetermined branching ratio.
A conventional PLC coupler/splitter device is manufactured by the following process. Firstly, for example, the pattern for the Y-shaped waveguide core 4 is designed. A photomask is then prepared based on this pattern, and the photomask pattern is transferred using photolithography and the Y-shaped waveguide core 4 is formed. However, because this photomask is expensive, it has not been possible to prepare photomasks for a variety of characteristics.
Furthermore, it has not been possible to carry out the minute adjustments of the characteristics such as the branching ratio and the like in the manufacturing process using optical characteristics monitoring equipment, because the Y-shaped waveguide core 4 is invariably formed almost same with the photomask pattern.
Concerning other PLC devices such as an AWG or Mach-Zehnder interferometer, the same problem exists because the manufacturing process is the same.
In addition, the minute adjustments of the optical characteristics can be contributed to an improvement in the actual production yield not only during the manufacturing process but also after the completion of manufacturing. Therefore, technology has been looked for that enables minute adjustments of the optical characteristics of the PLC devices after the completion of manufacturing process. It can be useful not only for optical coupler/splitter devices, but also for other devices such as, for example, gain equalizers, dispersion compensators, and the like that use Mach-Zehnder interferometers.
The present invention was conceived in view of the above described circumstances and it is an aim thereof to provide a method for manufacturing optical couplers/splitters that allows ease of integration and allows the optical characteristics such as the branching ratio to be adjusted during the manufacturing, and to provide a method for adjusting the optical characteristics of PLC devices after the completion of manufacturing process.
To put it concretely, it is an aim of the present invention to provide a method for manufacturing optical couplers/splitters and a method for adjusting the optical characteristics of PLC devices that allow minute adjustments to be made to the optical characteristics both during the manufacturing and after the completion of manufacturing of PLC devices including PLC type couplers/splitters.
In addition, it is an aim of the present invention to provide a method for manufacturing optical couplers/splitters and a method for adjusting the optical characteristics of PLC devices that allow these problems to be solved at as low a cost as possible.